Chipboard and its manufacture

ABSTRACT

A method of manufacturing chipboard having a central layer of coarse chips and two outer layers of chips whose fibers are oriented in the plane of the board. The central layer is formed by using as chips therefor end-grain cut flake-like chips, the fibers of which are oriented in the direction of thickness of the chips, so that the chip fibers in the central layer are oriented substantially transversely to the plane of the board. The chipboard thus formed has low swelling with moisture and relatively light weight for its strength. Such chipboard also has relatively high bending strength and transverse strength.

The present invention relates to a method for the manufacture ofchipboard, or particle board, comprising a centre layer of coarse chipsand outer layers of chips whose fibres are positioned in the plane ofthe board.

Conventional chipboard comprised of a centre layer of coarse wood chipsand sandwiching outer layers of finer wood chips are generallycharacterised by a high density. This high density can be ascribedprimarily to the fact that the fibres in the centre layer are positionedparallel with the longitudinal axes of the chips, i.e. in a plane whichextends substantially parallel to the plane of the manufactured board,and that the chips during the compression step required to form gluejoints between the chips are compressed to an appreciable extent suchthat the density of the board will be substantially higher than theintrinsic density of the starting material. In the case of conventionalchipboard manufacture, this increase in density will reach about 50%,such as to obtain a characteristic profile which corresponds to a givenchipboard standard, for instance SIS 234 801.

A high density, however, results in chipboard of lower moisturestability, primarily thickness swelling. Furthermore, because of itshigh density, chipboard is normally considered to be heavy and difficultto handle. The manufacture of conventional chipboard also involves highcosts for starting materials, i.e. chips, and glue, in addition toenergy costs.

Against the background of the afore-described known technique, there isa desire for a method by means of which the density of chipboard can bereduced, and therewith a reduction in the consumption of startingmaterials, while retaining the same characteristic profile in general,or while improving said profile.

A method which touches partially upon this problem is described inSE-B-346 945. It is stated in this document (page 10, line 27 to page22, line 27) that the size of the chips, the direction of chip fibresand the positioning or orientation of the chips in the board influencesthe properties of the finished chipboard, for instance such propertiesas density, swelling, mechanical strength, etc. For example, it has beenfound that when the longitudinal axes of the chip fibres are locatedtransversely to the plane of the board, the board will be lesscompressible as a whole after being formed and the smaller particleslocated in the outer layers will be compressed to a greater extent thanthe particles located in the core of the board. For the purpose ofobtaining chipboard in which a greater percentage of fibres are locatedwith the longitudinal axes of the fibres extending transversely to or atan angle to the plane of the board, it has been proposed in accordancewith SE-B-346 945 to use very short, essentially cubic chips asconventional wood chips, which because of their short lengths can bepositioned with the long axes of the fibres extending both verticallyand horizontally and in positions there between. There is obtained inthis way random orientation of fibres in all directions.

For the purpose of reducing density and consumption of startingmaterials still further, while retaining the conventional, usefulproperties of the finished chipboard, or even improving the level ofsuch properties, there is proposed in accordance with the invention amethod which will impart a more refined and positive orientation of thechip fibres in the centre layer in a direction perpendicular to theplane of the finished board. To this end, it is proposed in accordancewith the present invention that the chips used for the centre layer ofthe board are end-grain cut, flake-like chips whose fibres are orientedin the thickness direction of said chips, the chips fibres in the centrelayer of the board being orientated substantially transversely to theplane of the board. By flake-shaped is meant here the shape of a bodywhose width and length are substantially greater than the thickness ofthe body. When using such flake-shaped chips, the chips will settlenaturally on their respective base or top surfaces when forming thecentre layer, the longitudinal axis of the fibres being orientedessentially transversely to the plane of the board. Due to thesubstantially unitary transverse direction of the fibres in the centrelayer, a very high resistance to compression will be encountered in saidlayer during the compression stage, and consequently the conventionaltype chip particles which form the outer layers and whose fibres areorientated in the plane of the board are compressed to relatively thinlayers of high density, whereas the volumetrically larger centre layerobtains a relatively low density. Tests have shown that when practicingthe method proposed in accordance with the invention, there can beobtained three-ply chipboard whose total density is lower than thedensity of conventional chipboard, while retaining or improving theintrinsic or fundamental characteristic profile of the board.Consequently, chipboard manufactured in accordance with the novel methodwill require a smaller quantity of wood starting materials thanconventional chipboard manufactures. This lower consumption of woodstarting materials also decreases costs for glue and energy in themanufacture of said board. The resultant high mass surface density ofthe outer layers provides denser surfaces, which, for instance, decreasepaint and varnish consumption when treating the surfaces of the boardand enable the board to be lined with thinner paper liners.

The invention also relates to chipboard, or particle board, manufacturedin accordance with the method and comprising a centre layer of coarsechips and outer layers of chips whose fibres are oriented in the planeof the board, the chips in the centre layer comprising end-grain cut,flake-shaped chips whose fibres are oriented in the thickness directionof the chips, the chip fibres in the centre layer being orientedsubstantially transversely to the plane of the board.

The invention will now be described in more detail with reference to theaccompanying drawing, in which

FIG. 1 is a schematic, longitudinal sectional view of conventionalthree-ply chipboard with the chip fibres extending substantiallyparallel to the plane of the board;

FIG. 2 is a schematic, longitudinal sectional view of three-plychipboard manufactured in accordance with the present invention, withthe chip fibres of the centre layer oriented substantially at rightangles to the plane of the board; and

FIG. 3 is a schematic plan view of apparatus for cutting endgrain chipsintended for forming the centre layer chips of chipboard constructed inaccordance with the invention.

The conventional three-ply chipboard illustrated in FIG. 1 consists of acentre layer 10 comprised of chips 12, the fibres of which are orientedin planes which extend substantially parallel with the plane of thefinished board, as illustrated by lines 14. Two outer layers 16 containfiner chips, so as to provide a finer and denser surface structuresubsequent to being pressed, the density of the outer layers beinghigher than the density of the centre layer. Board manufactured in thisway will generally have a relatively high total density, resulting inheavy board, while the particular orientation of the fibres in thecentre layer renders the board sensitive to moisture, which ismanifested primarily in swelling of the board in the direction of itsthickness. One reason for the high density of the centre layer is thatthe chips with fibres extending parallel to the plane of the board canbe readily compressed, which means that a large quantity of woodstarting material must be used in order to produce chipboard of giventhickness and given acceptable fundamental characteristic profile.

The inventive chipboard, illustrated schematically in section in FIG. 2,consists of a centre layer 18 and two outer layers 20. In the case ofthe inventive chipboard, however, the centre layer 18 is composed ofend-grain cut, flake-shaped chips 22, the fibres of which are orientedin the thickness direction of the chips, as illustrated by the lines 24.

As indicated above, the invention is based on the concept of utilizingthe inherent resistance to compression of the wood chips, in a manner toachieve a reduction in the density of the finished board. This presumesan alternative method of producing the chips and of positioning thechips in the centre layer of the board. Thus, there is required aconvertible starting material, such as round wood, slabs and edgings. Inorder to obtain a unitary chip fibre direction essentially transverselyto the plane of the board when forming the centre layer, the chips 22need to be end-grain cut chips which are so configured that, duringforming of the centre layer, the chips will position themselves suchthat the largest dimension of the chips will lie parallel with the planeof the board. It has been found in practice that flake-like chips ordisk-shaped chips are extremely well suited for this purpose.

FIG. 3 illustrates schematically an apparatus for producing end-graincut chips suitable for use in the centre layer 18. Reference is madebelow to this apparatus in conjunction with a description of testscarried out on a laboratory scale in a comparison study between, on onehand, reference chipboard of conventional composition and manufacture,and, on the other hand, chipboard manufactured in accordance with theinventive method, this chipboard having a centre layer composed offlake-like end-grain cut chips and embraced by more dense, compressedouter layers.

Tests

An assortment of industrially produced chips were used as chip materialin reference board and for the outer layers in end-grain board. Thecentre layer chips of the reference boards were knife cut with the fibredirection parallel with the longitudinal axis of the chips, the chipshaving a maximum length of about 30 mm.

The chips had an estimated maximum thickness of 2 mm. The outer chiplayer comprised fine chips which fell within the fraction-compositionused in the manufacture of furniture board having fine-chip outerlayers.

The end-grain chips were produced from sawn, undried spruce planksmeasuring 65×155 mm.

Chip manufacture

In the manufacture of end-grain cut chips for the centre layer of theinventive chipboard, the planks 26 (FIG. 3) were cut to a length of 90mm measured in the fibre direction. Chip cutting was effected with theaid of a rotating disk 28 having a diameter of 815 mm and provided with8 knives (not shown) on one end side. The disk rotated at a speed ofabout 900 r.p.m. The following tool angles were measured: Rake angleγ=45°, edge angle β=35°, relief angle α=10°. The knife setting, i.e. thedistance between the flat disk and the knife edge, was selected at 1.0mm. This setting corresponds to a nominal chip thickness of about 1.0mm. In the manufacture of chips, the planks 26 were placed in atransport chute with the year rings of the planks facing towards theknife-carrying disk 28. The planks 26 were advanced by means of drivenpress wheels 30 which urged the planks against the disk. The resultantend-grain chips thus produced were then dried and fractionated bypassing the chips through a flat laboratory screen provided withsquare-mesh wire screen inserts. The result is set forth in Table 1below.

                  TABLE 1                                                         ______________________________________                                        Fractional composition of the end-grain cut                                   chips                                                                                      Screen dimensions                                                                          Percentage                                          Fraction     (mm)         (%)                                                 ______________________________________                                            I        >8.0         5.8                                                   II         8.0-1.0      88.5                                                III          <1.0         5.7                                                 ______________________________________                                    

The fractions I and III were excluded in the subsequent boardmanufacturing process. Thus, solely fraction II was used. In thisrespect, the chips in fraction I can be made smaller and the chips infraction III can be incorporated with the assortment of chips forproducing outer layers in the industrial manufacture of chipboard.

Chipboard manufacture

Three-ply chipboard was manufactured at a nominal thickness of 20 mm anda density of 600 kg/m³. The centre layer constituted 60% of thethickness of the chipboard, whereas the outer layers constituted 40% ofsaid thickness. For the purpose of studying the influence of layerdensity on the characteristic profile of the board, four density regionswere selected for the centre and outer layers respectively.

Table 2 below discloses information concerning the nominal and measuredlayer density, the measured layer thicknesses in mm, the calculatedlayer distribution and chipboard designations. Reference chipboard wasmanufactured solely from conventional industrial chip assortments, andis referenced R.

                                      TABLE 2                                     __________________________________________________________________________    Nominal and measured density of surface and                                   centre layers, and layer thicknesses.                                         Nominal       Measured  Measured layer                                        density       density   thickness Distribution                                    Surface                                                                            Centre                                                                             Surface                                                                            Centre                                                                             Surface                                                                            Centre                                                                             Surface/                                    Board                                                                             layer                                                                              layer                                                                              layer                                                                              layer                                                                              layer                                                                              layer                                                                              Centre layer                                No. (kg/m.sup.3)                                                                       (kg/m.sup.3)                                                                       (kg/m.sup.3)                                                                       (kg/m.sup.3)                                                                       (mm) (mm) (%)                                         __________________________________________________________________________    1.sup.                                                                            825  450  960  400  3.3  13.0 34/66                                       1R  825  450  780  480  4.1  11.0 43/57                                       2.sup.                                                                            750  500  960  410  2.9  13.8 30/70                                       2R  750  500  790  450  4.0  11.3 41/59                                       3.sup.                                                                            675  550  980  400  2.6  14.5 27/73                                       3R  675  550  780  460  3.8  11.6 40/60                                       4.sup.                                                                            600  600  910  430  2.4  14.8 24/76                                       4R  600  600  810  500  2.6  14.2 27/73                                       __________________________________________________________________________

The difference between desired nominal board density and the measureddensity will be clearly seen from the table.

It will also be seen from Table 2 that board manufactured from end-graincentre-layer chips in accordance with the invention achieves the desiredsurface distribution only at a low centre layer density (board 1).

All reference board manufactured from industrially used chipassortments, with the exception of board 4R, achieve the desired layerdistribution 40/60.

The method in which the end-grain cut chips intended for the centrelayer (i.e. the fibre direction) and the geometry of said chips(flake-shaped) are considered to have contributed to increasedcompression already in the low density regions.

The boards were hand-formed, sheet for sheet, in a forming box measuring300×300 mm. Pressing was effected under high pressure in a hot press ata temperature of 180° C. Press plates and spacer strips were used in thepressing operation. The press closing time, i.e. the time lapse betweenupper press-plate contact and spacer strip contact, was very short inthe case of the reference boards, more specifically an average timelapse of 10 seconds. Corresponding boards having centre layerscomprising end-grain chips engendered a compression resistance whichresulted in a compression time of 30-40 seconds.

The influence of press closing time and compression resistance on layerthickness and layer density can be seen from Table 3 below, whichdiscloses the density factor. By density factor is meant here the ratioof the surface layer density to the centre layer density. The factor isgiven both for the nominal density values and for the measured values.The table also includes compression, i.e. the ratio of measured andnominal factors (increase and decrease in layer density). The tableshows a marked increase in compression with increased centre layerdensity of board manufactured from endgrain, centre-layer chips inaccordance with the invention. On the other hand, reference boards showa decrease (1R) and a small increase (2R and 3R) in compression. Board4R having the highest centre-layer density also exhibits the greatestincrease in compression (Table 3), which corresponds substantially tothat of board 2 which has a lower centre-layer density of about 100kg/m³ (Table 2).

                  TABLE 3                                                         ______________________________________                                        Density factors and compression.                                                                       Compression                                                                   Increase (+)                                         Board     Density factor Decrease (-)                                         No.       Nominal  Measured  (%)                                              ______________________________________                                        1.sup.    1.83     2.40      +31                                              1R        1.83     1.63      -11                                              2.sup.    1.50     2.34      +56                                              2R        1.50     1.76      +17                                              3.sup.    1.23     2.45      +99                                              3R        1.23     1.70      +38                                              4.sup.    1        2.12      +112                                             4R        1        1.62      +62                                              ______________________________________                                    

Testing of inherent properties

The mechanical strength properties of the boards were tested inaccordance with Swedish chipboard standards (SIS 234801). Four samplebodies were taken from each board, for the purpose of determining thebending strength of the board. Two test bodies were then taken from theaforesaid test bodies and tested for transverse tensile strength, eachbody being placed around the fracture location.

The test carried out on the dimensional stability of the boards wasrestricted to investigating swelling of the boards in the direction oftheir thicknesses, subsequent to being submerged in water for 2 hoursand 24 hours respectively. Ten test bodies from each board were includedin this test. It can be mentioned that all boards had been rubbed downwith an abrasive prior to being tested. The prevailing density of eachtest body was also determined.

Results

The results obtained when testing the intrinsic properties of the boardare set forth in Table 4 below. The table shows the measuredcharacteristic properties as a mean value x with associated standarddeviations s for each individual chipboard. Within each density range(combination of surface density and centre layer density) chipboardcomprising end-grain centre layers was compared with reference chipboardwhose centre layers comprised conventional industrial chips.

                                      TABLE 4                                     __________________________________________________________________________    Results obtained when determining characteristic properties - transverse      tensile strength, bending strength and thickness swelling.                    Transverse tensile strength                                                                      Bending strength                                                                             Thickness swelling                          Board         density        density                                                                            2 h   24    density                         No. - x (MPa)                                                                          s (MPa)                                                                            (kg/m.sup.3)                                                                       - x (MPa)                                                                          s (MPa)                                                                            (kg/m.sup.3)                                                                       - x(%)                                                                           s(%)                                                                             - x(%)                                                                           s(%)                                                                             (kg/m.sup.3)                    __________________________________________________________________________    1.sup.                                                                            0.57 0.05 586  20.80                                                                              2.37 584  13.5                                                                             0.7                                                                              19.0                                                                             1.3                                                                              577                             1R  0.44 0.07 603  17.71                                                                              1.55 605  17.5                                                                             0.7                                                                              20.5                                                                             1.0                                                                              605                             2.sup.                                                                            0.66 0.06 583  19.69                                                                              2.40 584  13.8                                                                             0.8                                                                              19.9                                                                             1.5                                                                              581                             2R  0.43 0.06 596  17.96                                                                              0.85 600  17.8                                                                             0.6                                                                              21.1                                                                             1.0                                                                              582                             3.sup.                                                                            0.66 0.08 574  16.69                                                                              1.26 572  13.5                                                                             0.9                                                                              21.1                                                                             1.6                                                                              566                             3R  0.49 0.05 581  16.09                                                                              1.90 588  16.6                                                                             0.4                                                                              19.8                                                                             0.6                                                                              573                             4.sup.                                                                            0.71 0.06 577  16.65                                                                              2.36 579  14.8                                                                             1.7                                                                              24.9                                                                             2.3                                                                              570                             4R  0.51 0.09 601  19.12                                                                              3.36 606  18.0                                                                             0.2                                                                              21.5                                                                             0.5                                                                              594                             __________________________________________________________________________

Transverse tensile strength

The mechanical strength of chipboard is density dependent. In the caseof chipboard having a conventional centre layer structure, i.e. astructure in which the chip fibres are positioned parallel with theplane of the board, the density of the centre layer is a criterion thetransverse tensile strength of the board. Test bodies drawn from thetransverse tensile strength test exhibit centre layer fractures, thusalso confirming the strength influencing function of the centre layer. Acloser study of the results obtained with the transverse tensilestrength test will show that board whose centre layers compriseend-grain cut chips have a lower centre-layer density throughout incomparison with corresponding reference board, irrespective of the boardtype (1-4) measured in accordance with Table 2. This reduction indensity is calculated as being 10-20%. The reduction in centre layerdensity resulted in an increase in the outer layer density of end-graincut board by 12-25%.

The tests showed that with a board density of 600 kg/m³, the end-grainboards had a transverse tensile strength of 0.5-0.70 MPa. Correspondingvalues for conventional reference boards were 0.45-0.50 MPa. Theend-grain boards had these last mentioned values at densities as low as525 kg/m³. Thus, the greater transverse tensile strength of end-grainboard can be utilized in decreasing the board density. The extent ofthis reduction, however, is limited by the lowest permitted strengthvalues.

Bending strength

Distinct from the centre-layer-density dependency of the transversetensile strength, the bending strength of board is highly dependent onthe density of the outer layers. As earlier established, the surfacelayer density of end-grain cut board is 12-25% higher than the surfacelayer density of reference board. This also implies higher strengthvalues in the case of bending or flexural loads. In the case ofend-grain cut board having a density of 600 kg/m³, the bending strengthis from 18-22 MPa, depending on layer density distribution.Corresponding values for reference board (R-board) is 17-18.5 MPa. Inthe case of the end-grain cut board 1 and 2, these reference boardvalues were achieved at densities as low as 550 kg/m³.

Depending on the lowest permitted bending strength values, the boarddensity of end-grain cut boards can be made about 50 kg/m³ lower thanconventionally manufactured board, while still achieving the samebending strength.

Thickness swelling

The centre layer density, and therewith compression, influences thebehaviour of chipboard in the presence of moisture. For instance,thickness swelling of board will increase with increased density, whichas a rule has been produced by greater compression. Various methods areavailable for restricting swelling, at least with respect to short-termswelling (storage in water for less than 2 hours). No swellinginhibiting methods were applied during the present investigation,however. Consequently, the absolute swelling values recorded can beunderstood as being very high.

Prior to carrying out the swelling tests, the moisture quotients of thetest bodies were measured, wherewith it was found that the test bodiestaken from end-grain board had a board moisture quotient of 4.0%.Corresponding values for the reference boards were 4.9%.

Table 4 shows the swelling values with associated centre layer density.In the case of end-grain boards 1-4, swelling was measured after storingthe test bodies in water for 2 hours, and was found to be on average13.9%. Corresponding average values for reference boards 1R-4R werefound to be 17.5%. The difference between the swelling of end-grainboard and reference board after being stored for 24 hours in water arepractically nonexistent. The higher centre layer density of the endgrainboards 3 and 4 engender higher swelling after 24 hours than the boards 1and 2. The relationship between density and swelling in the case of thereference boards is less distinct. With respect to swelling (2 and 24hours), the end-grain boards 1 and 2 offer advantages over thecorresponding reference boards 1R and 2R. Consequently, a compositionaccording to types 1 and 2 should be chosen for conceivable industrialmanufacture.

The afore-described tests carried out on mutually different kinds ofcentre-layer chips illustrates that the characteristic profile ofthree-ply chipboard can be influenced by the construction of the centrelayer. It can therewith be established that end-grain chips improve suchproperties as transverse tensile strength, bending strength andthickness swelling in a marked manner within a given board density rangecompared with conventionally manufactured chips, in which the chipfibres are oriented in the plane of the board. This fact can be utilizedto manufacture board of improved characteristic profile and/or inreducing the total board density. A reduction in board density canassist generally in achieving a reduction in costs, inter alia withrespect to wood, glue, energy, transportation, etc. Low weight chipboardis desirable to the user of such board, and such low weight chipboardcan be produced in accordance with the invention, as defined in thefollowing claims.

I claim:
 1. Chipboard comprising a central layer of coarse chips and twojuxtaposed outer layers of chips whose fibers are oriented in the planeof the board, wherein the chips in the central layer consist ofend-grain cut flake-like chips, the fibers of which are oriented in thedirection of thickness of the chips, whereby the chip fibers of thecentral layer are oriented substantially transversely to the plane ofthe board.
 2. In a method of manufacturing chipboard having a centrallayer of coarse chips and two outer layers of chips whose fibers areoriented in the plane of the board; the improvement comprising formingsaid central layer by using as chips therefor end-grain cut flake-likechips, the fibers of which are oriented in the direction of thickness ofthe chips, whereby the chip fibers in the central layer are orientedsubstantially transversely to the plane of the board.